This invention relates generally to turbofan gas turbine engines, and more specifically to load reduction systems used in support systems for the fan assemblies of such engines.
A turbofan gas turbine engine used for powering an aircraft in flight typically includes, in serial flow communication, a fan assembly, a low pressure compressor or booster, a high pressure compressor, a combustor, a high pressure turbine, and a low pressure turbine. The combustor generates combustion gases that are channeled in succession to the high pressure turbine where they are expanded to drive the high pressure turbine, and then to the low pressure turbine where they are further expanded to drive the low pressure turbine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan assembly and the booster via a second rotor shaft.
The fan assembly includes a plurality of circumferentially spaced apart fan blades extending radially outwardly from a rotor disk that is drivingly connected to the low pressure shaft. Each fan blade generally has an airfoil section and an integral dovetail root section that attaches the blade to the rotor disk. The fan assembly is rotatively supported on a nonrotatable frame, commonly referred to as the fan frame, by a support system that typical includes a number of bearings and bearing support structure.
During engine operation, there is a remote possibility that a foreign body, such as a bird, could impact the fan assembly and cause part or all of a fan blade to become detached from the rotor disk. Such a blade loss would create a large imbalance, which could result in the transmission of potentially damaging imbalance forces to the fan frame. To alleviate the transmission of such imbalance forces, it is known to provide the support system with a decoupler (also commonly referred to as a “fuse”) which is a frangible structure designed to fail in response to a predetermined load. Thus, in the event of a blade loss, the unbalanced rotation of the fan assembly will cause the decoupler (“fuse”) to fail such that substantial imbalance forces are not transmitted to the fan frame. Accordingly, use of a decoupler effectively reduces the overall weight of the engine because the fan frame and related structure need not be made sufficiently strong to withstand substantial imbalance forces. This structural decoupling will also reduce the stiffness of the fan assembly support system and hence will decrease the natural frequency of the fan assembly.
Although standard procedure is to quickly shut down the engine in the rare event of a blade loss, the fan assembly will continue to rotate due to windmilling caused by the forward motion of the engine. As the fan assembly slows down to the lower windmilling speed, “recoupling” of the fan assembly and the fan frame will occur through the contact load paths between the booster rotor blades and the booster stator and/or between the fan blades and the fan casing. Such recoupling will provide a new transmission path to the fan frame for the imbalance forces, resulting in undesirable engine and/or airframe vibration. This vibration can be particularly troublesome if the depressed natural frequency of the fan assembly is equal to or close to the windmilling speed because of the resulting resonant or slightly off-resonant operation.
The conventional load reduction devices (LRDs) are designed to provide limited decoupling at high speed in order to place the decoupled mode natural frequency of the fan rotor system above the windmilling speed of the engine. The conventional load reduction devices provide limited fan rotor decoupling whereby the fan rotor forward bearing (“No. 1 Bearing”) is fused and the fan case aft bearing (“No. 2 bearing”) is only partially fused. In conventional designs, further load reductions by additional decoupling of the load paths at additional bearing locations such as the No. 2 bearing location, has not been possible in order to maintain sufficient residual stiffness to maintain decoupled mode natural frequency above the windmilling speed to obtain stable operation with acceptable response levels during windmilling operation.
Accordingly, it would be desirable to have a fan assembly support system which reduces in-flight engine vibration at windmilling speeds after a fan blade damage induced structural decoupling of the fan rotor from its support system. It is desirable to have a tuned and damped load reduction device and system that permits increased decoupling and increased load reduction at high speeds. It is desirable to have a tuned and damped decoupling and load reduction system which enables windmilling operation with no speed restriction.